A polymer electrolyte membrane fuel cell (PEMFC) includes a stack, a reformer, a fuel tank, and a fuel pump. In the PEMFC, the fuel in the fuel tank is supplied to the reformer by an operation of the fuel pump. The reformer reforms the fuel to generate hydrogen gas. The stack causes the hydrogen gas supplied from the reformer and oxygen to electrochemically react with each other to generate electric energy.
A direct methanol fuel cell (DMFC) is similar to the PEMFC, but the DMFC can directly supply liquid methanol fuel to its stack. Since the DMFC does not need to use the reformer that is in the PEMFC, the DMFC can be small in size.
The DMFC is provided with a membrane-electrode assembly that includes a polymer electrode membrane, and anode and cathode electrodes attached to the opposite sides of the polymer electrode membrane. Further, the DMFC can include a separator to form a stack. In a DMFC stack, tens to hundreds of membrane-electrode assemblies, in which an electrochemical reaction arises, are stacked, and opposite end plates are compressed by a coupling bar, air pressure or the like to reduce contact resistance between component elements.
A DMFC system includes a device to support and control an operation of the DMFC stack. Such a device generally includes an auxiliary component (balance of plant: BOP). The balance of plant can include a reactant supply to serve fuel and an oxidant; a controller to control the system; a power converter; a temperature adjuster; a humidifier; a sensor; a monitoring device; etc.
Also, the DMFC system includes an electricity storage device that includes at least one of a battery, a super capacitor, etc. and a controller to control it to be charged and discharged; and various electric devices that operate by the electric energy. The electric devices include a blower, a pump, a compressor, a regulator, a sensor, a powered valve, an electronic device, an electronic circuit, etc. These electric devices operate by receiving power from a commercial power source or the electricity storage device before receiving the power from the DMFC system.
The DMFC has a problem, however, in that a catalyst may be poisoned by carbon monoxide (CO) generated when unreacted fuel remaining in the anode moves to the cathode via the electrolyte membrane and reacts with oxygen in a cathode catalyst layer because the fuel remains in a fuel cell main body having a structure, such as an active type stack, a semi-passive type cell pack or the like when the fuel cell system stops operating. Further, the fuel remaining in the anode includes carbon dioxide generated by reaction in the anode, so that carbon dioxide can have an adverse effect on the membrane-electrode assembly. Most of the polymer electrolyte membranes, which have recently been widely used, greatly expand while being dipped in methanol. Therefore, in the case where the DMFC system stops operating while the methanol supplied as the fuel remains in the fuel cell main body, the electrolyte membrane is damaged. Further, when the DMFC system is restarted in this state, the performance of the system is largely deteriorated and the lifetime of the fuel cell is notably shortened. To prevent the membrane-electrode assembly from corrosion and damage, there is conventional technology to discharge or exhaust the methanol fuel remaining in the fuel cell main body.
For example, the DMFC system stops supplying the liquid fuel such as methanol to the stack when it stops operating; supplies an oxidant to the stack for a predetermined period; and stops supplying the oxidant after exhausting the remaining fuel. However, the conventional driving method is performed only when the molar concentration of methanol ranges from 1 M to 6 M. Therefore, in the conventional driving method, the membrane-electrode assembly is still likely to be corroded or damaged by the fuel remaining in the stack.
Thus, in the conventional DMFC system cycling between operation and a stop in operation (a “stop”), as the frequency that the DMFC system stops increases, the performance of the stack decreases, as shown in FIG. 1.